IMO, one can make the argument that Colonel Sun, Devil May Care, Solo and Trigger Mortis all take place in the same universe/timeline as Fleming's books. I would call them all part of the canon, although especially CS.
I would argue that Colonel Sun has much more of a canonical status than any of the subsequent continuation novels. There are several reasons for this.
Firstly, Kingsley Amis was the only continuation author who had been involved in the work with one of Ian Fleming's own Bond books (TMWTGG - although his notes weren't used) and who had had personal contact with him (Fleming had given Amis' The James Bond Dossier his blessing before he died according to the foreword i the CS paperback from '91). Therefore i believe that Amis had a sort of special insight into Fleming's writing.
The fact that CS was published in 1968, and thus was written in the same time period as the original novels also means that it has a special status. Other continuation novels have either moved Bond to the contemporary times in which the books were written (80s for Gardner, 90s for Benson and 00's for Deaver) or become some sort of "historical novels" when books written in the 21st century (Faulks, Boyd, Horowitz) take place in the 1960s. So either the time setting has changed and therefore Bond has become something of a different character, or he has been left in the 60's, but written from a contemporary perspective. That is not the case with Colonel Sun, and this makes the book more authentic to me.
This is also the only continuation novel that was made into a comic strip in the Daily Express, by Jim Lawrence and Yaroslav Horak, who also adapted the later Fleming stories and treated CS as a natural continuation of these.
Furthermore, this is the continuation novel which has come closest to be filmed as part of the official EON series. Harry Saltzman had been involved in the making of Jeoffrey Jenkins' novel Per Fine Ounce. When Glidrose chose to publish Colonel Sun instead of Jenkins's book Saltzman dismissed the idea of ever turning Amis' book into a film. When Saltzman left the film series, there was talk of using Amis' book as the basis for a film. Amis, however, had several times publicly badmouthed Roger Moore's Bond films, which Broccoli didn't appreciate and he was therefore reluctant to film the book. (source
Despite this, elements from the book have been used in a few Bond films; M's kidnapping and imprisonment on an island (TWINE) and the title character was featured in DAD. (Although his name was changed to Colonel Tan-Sun Moon instead of Colonel Sun Liang-tan... but come on!).
So because of these reasons IMO CS is as canonical as Fleming's books.
The other books by Faulks, Boyd and Horowitz I feel could be viewed as canonical, unless there is blatant contradictions and inconsistencies when compared to Fleming's books and CS. I know some people think that Devil May Care contradicts CS, but I have no memory of this. I think it's OK if one book doesn't mention the events of another, as long as it isn't too obvious a contradiction (even Fleming contradicted himself sometimes - for example about the location of Royale-les-Eaux
Therefore I regard DMC, Solo and TM as taking place in the same timeline/universe as the Fleming books and CS. Even some small contradictions are easier to swallow than the age-less, eternally youthful Bond of the Gardner/Benson books.